The invention concerns a method for recognition of the variator slip in continuously variable transmissions (CVT transmissions).
CVT transmissions have a variator for continuously variable adjustment of the ratio. A customary structure is a belt drive variator having two pairs of beveled pulleys and rotating a torque-transmitter element therein, such as a pushing linked band or a chain. The beveled pulleys are loaded with pressure from the transmission oil pump in order, on one hand, to actuate the ratio adjustment and, on the other, to ensure a contact pressure needed for transmission of the torque upon the belt drive element.
Another usual structure is a swash plate variator in semi-toroidal or fully toroidal design.
An efficient operation of the variator requires a contact pressure calculation as exact as possible and that satisfies the requirements. The contact pressure needed for a reliable torque transmission is, in essence, a function of the variator geometry, of the friction ratios between band and beveled pulleys, the applied transmission input torque of the rotational speed of the pulleys and of the variator ratio and is usually calculated by an electronic transmission control of the CVT, according to the operating point.
The electronic transmission control receives the torque information, for ex., via a torque sensor inside the transmission on the variator or via an engine torque signal of the electronic engine control (for ex., as CAN signal). Both methods are relatively inaccurate, especially in dynamic operation, with the consequence of that over tightening of the variator is detrimental to the degree of efficiency.
DE-A 44 11 628 has disclosed a method in which the contact pressure of a belt drive variator is adjusted via a slip regulation of the belt drive means. At the same time, the variator slip is determined by means of rotational speed sensors disposed respectively on a primary pulley and a second pulley of the variator.
From DE-A 41 38 603 has become known to determine indirectly the operation state of a vehicle equipped with automatic transmission and lock-up clutch by means of a sensor of solid borne sound situated on the rear axle suspension of the transmission in order then to control the slip of the lock-up clutch so as to prevent humming noises and vehicle jolt phenomena. The disadvantage of this method is that the values determined are often inaccurate. The inaccuracy is not relevant for the slip control of a converter lock-up clutch since, in the case of doubt, the slip having been set too high is borne without problem by the lock-up clutch. Thus, it is not possible to apply the method without problem to a variator where, on one hand, the slip has to be prevented and, on the other, the contact pressure level has to be kept as close as possible to the slip limit.
Therefore, this invention is based on the problem of outlining a method which makes possible the control of the variator pressure as needed by detecting the exact variator slip.
It is accordingly proposed to detect the variator slip by monitoring and estimating the rotating variator according to vibration and by a subsequent evaluation.
The inventive solution is shown by way of example with reference to a belt drive variator having a pushing linked band. But the range of application of the invention extends to all types of variators for mechanical torque transmission.
The present invention is directed to a method for detection of variator slip in a continuously variable transmission by monitoring a vibration generated by a rotation of the continuously variable transmission and determining a tribologic slip of the continuously variable transmission by evaluation of the monitored vibration.